Oxide coated cathode



April 30, 1935.

W. W. ElTEL ET AL OXIDE COATED CATHODE Filed Feb. 25, 1935 'i i i ii INVENTORS WILL/AM W. E/TEL a JACK McCULLOUGl-l.

A TTORNE Y Patented Apr. 30, 1935 OXIDE COATED CATHODE 1 William W; Eitel, San-Bruno, and Jack McCulr lough, Palo Alto, Calif., assignors to Heintz &

Kaufman, Ltd., San Francisco,

' ration of Nevada Application February 25 1933, Serial N0. 658,524

4.,Clai ns. (01. 250-275) Our invention relates to an oxide-coated cathode for thermionic tubes, and more particularly to such a cathode which has been formed into a predetermined shape by a stretching or deform- 5 ing process after the oxide layer has been applied. h 7

Among the objects in our invention are: To

provide an oxide-coated filament or cathode of high efiiciency; to provide a method of preforming an oxide-coated filament to a predetermined shape before insertion into a'thermionic tube; to'provide a thermionic tube-havinga stretched oxide-coated filament; and to provide a relatively rigid pleated ribbon filamenthaving an oxide coating which will maintain its shape under operating conditions in a thermionic tube. Other objects of our'invention willbe apparent or will be specifically pointed out in the description forming a part of this specification, but we do not limit ourselves to the embodiment of our invention herein described, asvarious forms may be adopted within the scope of the claims.

Referring to the drawing: r

Figure 1 is a longitudinal view of a thermionic tube embodying this invention, the envelope being cut away to expose the electrodes.

Figure 2 is a sectional view taken by the line 22 in Figure l. g 7 7 I Figure 3 is a View inelevation of a device used for cold stretching and forming the cathode to a predetermined shape.

When oxide-coated filaments were first developed, a core-material of platinum was commonly used, and oxides, usually a mixture of barium and strontium, were baked on to the core in a plurality of layers. Later nickel was substituted for platinum, but in all casesafter coating the core material'could not be deformed, as the bond between the oxide layer and the core was poor, and the oxides would flake off. Nickel, and its alloys, or other similar core materials anneal and become soft during theprocess of baking on the oxides, and when the completed cathode is mounted in a thermionic tube in such a condition, it is limp, lacks rigidity, and will stretch under spring tension. In addition, the annealed cathodes when self-supported either wholly or partially, in a position where the force of gravity may act on them, will gradually sag due to this stretch.

Any serious attempts heretofore to preform the filament after the oxide coating has been applied have been failures, as the oxide coating would scale off the core when the core was distorted.

as. indicated In certain cases, such as the bend at the top of,

cap ll.

Calif., a corpo- PATENT OFFICE the usual type of hairpin'filament, this cracking ofi did no harm and could be tolerated, as the loss of emitting material did not occur in the active portion of the filament. In other cases thecom pleted filament could not be formed at all after the oxide coating was put on and recourse was had to forming the core into the desired s hape,

dipping the formed core into a solution of oxides and baking the formed cathode. This still left the formed filament in an-annealed and soft condition," and the formedfilament'had to be handled and mounted with extreme care to prevent peeling of the oxides.

We have found that oxide coatedfilaments having a denseadherent coating such; as that described. in the application of J ack' McCullough, Serial No. 614,629, filed May31, l932jddnotapeel, flake or otherwise. lose their oxide coating when the core is distorted, and this invention broadly speaking comprises providing a: core, preferably of-nickel, with aeoating or layer of oxides as described in the above-mentioned application, and then stretching "and-bending the completed filament over a form to harden and set the filaone endr'by' a reentrant stem 2 through which aresealed two cathode leads 4 and which sup- 'ports',-"in this case three dummy supports 5; A

pleated ribbon filament 6 is welded at. its ends to the cathode leads, and the cross bar of every other pleat is supported by refractory support wires 7 loosely looped around the bars.

apart and .prevent vibration of the complete filament.

An anode 9 is sealed through a projection 10 in the upper portion of thefenvelope, and the exterior anode lead is connected to an anode A base I! is sealed on the stem end of the envelope, and the two cathode leads are connected to cathode prongs l4 insulated from the base by washers I5.

Thecathode is prepared for insertion in the.

7 These supportwires space the pleats at equal distances tube by first coating a core, preferably of nickel ribbon with an adherent layer of oxides, or similar electron-emitting material, and preferably by the use of the method andmaterial disclosed in the above-mentioned application of Jack Mc- Cullough. h

The coated ribbon is then stretched and'bent into a predetermined form, preferably while cold,

by the use of the stretching and forming device shown in Figure 3.

A foundation bar I 6 is provided with two round studs ll, one at each end, and is drilled and tapped in the center to receive a screw l9, provided at its outer end with a knurled handle 20, and at the other end with an unthreaded projection 2|, separated from the threads of the screw by a shoulder 22. A movable bar 23 is drilled to slide over the studs l1 and also to receive the projection 2|. A retainer collar 24 I is then passed over the end of the projection 2| and fixed in place by pin 25. The movable bar will then slide on the studs in'response to:the turning of the screw I9.

Each stud is provided with'an adjustablestop 26 and a terminal nut 21.

Equally spaced along each bar are ribbonpins 29, those on the foundation bar being preferably offset in position from'those of themovable bar. Removable splitpins 3B are positioned at-pne end of one bar and'at the opposite end of the other bar. These pins are; preferably tapered to grip the ribbon.

Qne end of the coated ribbon is placed in one split pin and the pin driven into its hole to grip the ribbon securely. r

The ribbon is then wound loosely aroundalternate pins, first on' one-bar and then'on the other, and. the free end of the ribbon secured in the remaining split pin. The screw is then rotated to take up the slack, and then further rotated to stretch the ribbon. The adjustable stops 26 are so positioned that'given lengths of ribbon may be uniformlystretched, andto prevent stretching beyond the. elastic limit'of the core material.

The tension is then relieved, the split pins removed, and the finished pleated filament removed from. the frame. It will be found that thecore material has been, hardened by the stretching and bending process, and that the filament has become set into the form defined-by the location and number of the pins. The formed filament may then be handled without excessive care, and

mounted on the ,cathodeleads;

It may be desirable, however, to use the'stretching frame as a holder for the filament, and the filament may be welded to thecathode leads while still on the frame, and then removed. 1

further deform in use, andmuch more precisely shaped cathodes may befas'hioned than when annealed ribbons are used. The emitting properties of the stretched filament have not been diminished by the stretching process, but on the contrary there appears to be some evidence of an increased efficiency, the reason for which we are not at present able to state.

It is well known that bending as well as stretching will harden'and set a metal. In the stretching frame as described, the bars-of the pleats are stretched, but the end loops are bent rather than stretched, although the outer circumference necessarily has to be stretched to form the bend. It'is therefore possible to carry out the spirit of the invention by bending alone, as such bending will give the same result as longitudinal extension.

This feature'is of. great financial advantage in the formation of cathodes from sheet core material, as for example the cylindrical cathodes for heater type electron tubes. The usual procedure is to blank and form the core material, mount andfinally coat and bake on the oxide layer. Our invention permits the coating to be done first on the sheet stock, the blanking and forming into a cylinderbeing done after the oxide layer has been applied. The forming into a-cylinder will harden the core material and give all the advantages here. disclosed fora ribbon type filament. I

Weclaimz" 7 7 p 1. The method of forming an electron-emitting cathode, which comprises coating a metallic ribbon with an adherent layer of electron-emitting material, and subsequently cold stretching said ribbonv throughout its length into the desired form.

2. The method of forming an electron-emitting cathode, which comprises coating 9. ductile ribbon with: an adherent layer of electron-emitting oxides, and subsequently cold stretching all of said ribbon into the desired form.

3. The method of forming an electron-emitting cathode which comprises coating a metallic core with an adherent layer ofelectron-emitting mate rial, and subsequently deforming all of said core into the desired shape. V,

4. The method of forming an electron emitting cathode which comprises coating an elastic metallic core witha non-elastic layer of electron emitting. material and subsequently cold stretching all of said core into thedesired form.

WILLIAM w. EITEL. JACK McCULLOUGI-I. 

